1. Field of the Invention
The present invention relates to the Digital Subscriber Line (DSL) Systems and specifically to back channel signaling in the DSL system.
2. Related Art
High-bandwidth systems, including DSL systems, use baseband modulation, single-carrier modulation as well as multi-carrier modulation schemes. Both DSL and other high-bandwidth systems such as wireless use modulation schemes such as Quadrature Amplitude Modulation (QAM), Carrier-less Amplitude and Phase Modulation (CAP), and Discrete Multi-tone (DMT) for wired media and Orthogonal Frequency Division Multiplexing (OFDM) for wireless communication. One advantage of such schemes is that they are suited for high-bandwidth applications of 2 Mbps or higher upstream (subscriber to provider) and 8 Mbps or higher downstream (provider to subscriber). Quadrature Phase Shift Keying (QPSK) utilizes quadrature carrier phase shift keying to encode 2 bits of information on a carrier frequency by employing waves in the same carrier frequency shifted by increments of 90°, which can be thought of as sine and cosine waves of the same frequency. Since the sine and cosine waves are orthogonal, data can be encoded in the amplitudes of the sine and cosine waves. Therefore, 2 bits can be sent over a single frequency using the quadrature keying.
CAP is similar to QAM. For transmission in each direction, CAP systems use two carriers of identical frequency above the 4 kHz voice band, one shifted 90° relative to the other. CAP also uses a constellation to encode bits at the transmitter and to decode bits at the receiver. A constellation encoder maps a bit pattern of a known length to a sinusoid wave of a specified magnitude and phase. Conceptually, a sinusoidal wave can be viewed to be in one-to-one correspondence with a complex number where the phase of the sinusoidal is the argument (angle) of the complex number, and the magnitude of the sinusoidal wave is the amplitude or modulus of the complex number, which in turn can be represented as a point on a real-imaginary plane. Points on the real-imaginary plane can have bit patterns associated with them, and this is referred to as a constellation and is known to one of ordinary skill in the art.
DMT modulation, sometimes called OFDM, builds on some of the ideas of QAM but, unlike QAM, it uses more than one constellation encoder where each encoder receives a set of bits that are encoded and outputs sinusoid waves of varying magnitudes and phases. However, different frequencies are used for each constellation encoder. The outputs from these different encoders are summed together and sent over a single channel for each direction of transmission. For example, common Asymmetric Digital Subscriber Line (ADSL) DMT systems divide the spectrum from 0 kHz to 1104 kHz into 256 narrow channels called bins (sometimes referred to as tones, DMT tones, or sub-carriers). These bins are 4.3125 kHz wide. The waveforms in each bin are completely separable from one another, i.e., orthogonal to each other. In order to maintain “orthogonality,” the frequencies of the sinusoids used in each bin should be multiples of a common frequency known as the fundamental frequency and in addition the symbol period τ, must be a multiple of the period of the fundamental frequency or a multiple thereof. In a DMT system, the number of bins is 256 for ADSL/ADSL2, 512 for ADSL2+ and 4096 for Very High Speed Digital Subscriber Line 2 (VDSL2). Each bin can carry a certain number of bits; this number can vary due to factors such as attenuation on the line, noise and crosstalk in the cable, and the transmit signal power spectral density (PSD). The aggregate bit pattern which comprises the bit patterns mapped to constellations in each of the bins during a symbol period is often referred to as a DMT symbol. For the purposes here, time domain references are often referred to in terms of DMT symbol periods also referred to as symbol periods.
FIG. 1 illustrates DSL communications layering in communications between central office (CO) 172 and customer premises equipment (CPE) 174. The communications has a downstream component from CO 172 to CPE 174 and an upstream from CPE 174 to CO 172 component.
Within CO 172 is layer 142, the Transport Protocol Specific-Transmission Convergence (TPS-TC) layer. At this layer support for application specific transports such as ATM or Ethernet are implemented. The function of the TPS-TC is to convert the payload data with appropriate framing into a bit rate for transport to the Physical Media Specific-Transmission Convergence (PMS-TC) layer. The gamma interface delineates the TPS-TC from the layer 2 applications above.
The next layer is layer 144, the PMS-TC layer. This layer manages framing, transmission, and error control over the line. In particular, this layer comprises the forward error correction (FEC) codes such as the Reed-Solomon (RS) Codes and interleaving. The interface between the TPS-TC and the PMS-TC layer is referred to as the alpha interface in the CO unit 172 and the beta interface in the CPE unit 174. It is very common that the transmit portion of the PMS-TC layer 144 comprises scrambler 102, RS encoder 104 and interleaver 106 to implement a RS code with interleaving (RS-ILV). DSL standards mandate the use of RS as the FEC.
The next layer is layer 146, the physical media dependent (PMD) layer. In the transmit portion, the PMD layer encodes, modulates and transmits data across physical links on the network. It also defines the network's physical signaling characteristics. In particular, constellation encoding and trellis codes map data into DMT symbols which are converted into time domain signals through the use of an IFFT where the time domain signal can be transmitted across subscriber line 160. The trellis code can also supply additional error correction. The interface between the PMD layer and the PMS-TC layer is referred to as the delta interface.
After processing by the PMD layer, the data is transmitted across subscriber line 160, where it is received by CPE 174 using its PMD layer 156. In a receiving capacity PMD layer 156 decodes, demodulates and recovers the data received across physical links on the network. Furthermore, PMS-TC layer 154 in a receiving capacity decodes data encoded by PMS-TC 144 and extracts data from the framing scheme. In particular it can comprise de-interleaver 108, RS decoder 110, and descrambler 112 to extract data encoded by PMS-TC 144. Finally, TPS-TC layer 152 is the transport protocol specific transmission conversion layer, configured for interfacing to specific transport protocols such as ATM or Ethernet.
In the upstream direction, CPE 174 transmits to CO 172. In the transmission, TPS-TC layer 152 functions in a similar fashion for CPE 174 as described for TPS-TC 142 in CO 172. Similarly PMS-TC 154 functions in a similar fashion as described for PMS-TC 144 and may additionally comprise scrambler 122 analogous to scrambler 102 in the downstream portion of TPC-TC 142, RS encoder 124 analogous to RS encoder 104 in the downstream portion of TPC-TC 142, and interleaver 126 analogous to interleaver 126 in the downstream portion of TPC-TC 142. PMD layer 156 functions in a similar fashion as described for PMD layer 146 in the downstream portion. The encoded and modulated signal is then transmitted from CPE 174 to CO 172 through subscriber line 160.
CO 172 in PMD layer 146, upon receiving the upstream encoded and modulated signal, performs the analogous decoding, demodulating and receiving of data across DSL to the downstream portion of PMD layer 156. The upstream portion of PMS-TC 144 functions similarly to the downstream portion of PMS-TC 156 and can comprise de-interleaver 128, RS decoder 130 and descrambler 132 analogous to de-interleaver 108, RS decoder 110, and descrambler 134, respectively. Finally, TPS-TC 142 functions in the upstream portion function similarly to TPS-TC 152 in the downstream portion.
More specifically, FIG. 2 illustrates a more detailed description of the transmission side of the PMS-TC layer (i.e., the downstream portion of PMS-TC 144 or the upstream portion of PMS-TC 154) as disclosed by present xDSL standards. Data from the TPS-TC layer transmitted to the PMS-TC layer can use one of two latency paths and one of two bearer channels. Input 202 represents data on the first bearer channel designated for latency path #0. Input 204 represents data on the second and optional bearer channel designated for latency path #0. Input 206 represents data on the first bearer channel optionally designated for latency path #1 when not designated in latency path #0. Input 208 represents data on the second bearer channel optionally designated for latency path #1. In addition, overhead data can be received by the PMS-TC layer including Embedded Operations Channel (EOC) 210, Indicator Bits (IB) Channel 212 and Network Timing Reference (NTR) 214, which can be combined by overhead multiplexer (MUX) 218. In addition, MUX 216 combines inputs 202 and 204, and MUX 220 as part of optional latency path #1 combines input 206 and 208. MUX 222 combines the output of MUX 216 with a sync byte and overhead data from MUX 218. Similarly MUX 224 combines the output of MUX 220 with an overhead sync byte and overhead data from MUX 218. Along each latency path, scrambler 226 and corresponding scrambler 228 scramble the data received from MUX 222 and MUX 224, respectively. FEC 230 and FEC 232 apply an FEC to the scrambled data from scramblers 226 and 228, respectively. Typically, the FEC used is a RS code and in the example of FIG. 1, FEC 230 and/or 232 can be RS encoder 104. Interleaver 106 of FIG. 1 comprises interleaver 234 and interleaver 236 which performs the interleaving of the encoded data received from FEC 230 and FEC 232, respectively. It should be noted that the FEC and interleaver parameters can generally be configured differently for each latency path. Finally, MUX 238 combines the encoded interleaved data for both latency paths to produce output 240 which is ready for processing by the PMD layer.
FIG. 3A illustrates a more detailed description of the transmission side of the PMD layer. Data is received as serial data by the PMD layer from the PMS-TC layer. This serial data is converted from a serial bit stream to parallel form by serial-to-parallel converter 302 for mapping into M parallel sub-channels, each sub-channel representing a specific sub-carrier. Each parallel sub-channel represents one of the bins or sub-carriers used, so for ADSL, serial-to-parallel converter 302 would produce 256 parallel sub-channels and for VDSL, serial-to-parallel converter 302 would produce 4096 parallel sub-channels. The various parallel bit sequences are passed to symbol mapper 304. Symbol mapper 304 maps each bit sequence into a constellation point within each sub-carrier (i.e., a DMT sub-symbol). For example, if the bit sequence relative to a specific sub-carrier comprises 3 bits, depending on the value of the three bits it is mapped to one of the eight points indicated by constellation 554 in FIG. 5. If the bit sequence comprises 5 bits, the bit sequence is mapped to one of the 32 points indicated by constellation 558. Furthermore, to increase robustness, the bit sequence can be transformed by a trellis encoder prior to the mapping onto a constellation point. The collection of the DMT sub-symbols mapped by symbol mapper 304 comprises the DMT symbol. Each subsystem is a complex number represented by a constellation point where the corresponding constellation represents points on the complex plane. As such inverse fast Fourier transform (IFFT) 306 becomes a modulator taking the collection of DMT sub-symbols as complex-valued numbers and provides M complex output samples which are converted to 2×M output real samples by taking the complex conjugates of the M samples. The parallel outputs of IFFT 306 are applied to parallel-to-serial converter 308 to provide a serial output signal. Essentially, the serial output signal is a digital time domain signal which undergoes additional time domain processing by time domain module 310 to produce a signal suitable for transmission of a subscriber line. Time domain module 310 can comprise components such as a cyclic prefix block, an up-sampler, and interpolator and a digital-to-analog converter to produce a continuous time domain signal.
FIG. 3B illustrates a more detailed description of the reception side of the PMD layer. An analog signal is received over the subscriber line where time domain module 332 provides a variety of time domain signal processing and can comprise components such as an analog gain module, an analog-to-digital converter and a digital gain adjustment module. The resultant digital time domain signal is converted to parallel sub-channels using serial-to-parallel converter 334. The parallel sub-channels are converted to frequency domain using FFT 336. Frequency equalizer 338 which has been previously trained during the initialization can apply selected gain to each frequency. The resultant frequency data corresponds to constellation points in a constellation associated with the sub-carrier corresponding to the sub-channel which is then unmapped by de-mapper 340 which can comprise a trellis code decoder back into digital data. The de-mapped digital data is then reassembled into a serial stream of bits by parallel-to-serial-converter 342 where the data is then handled by the PMS-TC layer.
While FIG. 3A shows an overview of the coding and modulation process taking place in the PMD layer, FIG. 4 shows the inclusion of a sync symbol. A sync symbol, which is not to be confused with the sync byte introduced in the PMS-TC layer which is used for overhead framing, is used to define the boundary of a DMT superframe. Typically, a sync symbol is followed by a predetermined number of data symbols. Together they define the DMT superframe. Typically, the superframe comprises a sync symbol and 256 data symbols. Characteristically, a sync symbol comprises 2-bits per available bin and is set to a well defined sequence of all ones or all zeros. A quadrant scrambler may then rotate the sync symbol as mapped onto the appropriate constellation. On the contrary, when user data is mapped on a DMT symbol, each bin has a certain number of bits which can be carried, the numbers vary due to factors such as line attenuation, noise and crosstalk in the cable, and the transmit signal PSD. In order for a bin to be useable, the signal-to-noise ratio (SNR) must be large enough to load 2 bits; alternatively, one bit of information may be loaded across multiple bins with lower SNR (referred as one bit constellations).
More specifically, in FIG. 4 between mapper 304 and IFFT 306 is MUX 404 which includes a sync symbol produced by sync symbol module 402 after a predetermined number of user data symbols have been processed. The sync symbol module 402 generates the sync symbol based on a well defined sequence of 2-bit symbols of all ones or all zeros and with the application of a quadrature scrambler.
During an initialization phase, SNR measurements are taken for each bin, which are used in that initialization phase to determine the number of bits that can be transmitted reliably over each bin. The process of determination is referred to as bit loading. To determine a bit loading profile, factors such as the transmit PSD, the ratio of useful receive signal power to total noise power, which includes background noise and other external noise sources such as crosstalk. FIG. 5 shows an example of a bit loading profile. For illustration purposes only 23 bins are shown in this example; bins 502, 504, 516, and 528 carry no data. Other bins carry varying amounts of data. Each combination of bits across all bins comprise a DMT symbol. Additionally, associated with each bin is a constellation for mapping bits onto the subcarrier. For example, bin 520 is only capable of carrying 2-bits, so mapper 304 maps 2-bits of data to a constellation point on constellation 552. Likewise, bin 522 is capable of carrying 5-bits, so mapper 304 maps a given 5-bits of data to a constellation point on constellation 558. In the same fashion bin 524 which can carry 3-bits is associated with constellation 554, and bin 526 which can carry 4-bits is associated with constellation 556. Each DSL standard defines the association of the number of bits with a particular constellation.
As mentioned previously crosstalk is a ubiquitous source of noise in a DSL system. FIG. 6 illustrates the various types of crosstalk typically experienced in a DSL system. For simplicity, CO 610 comprises two transceivers communicating over two subscriber lines to two CPEs. Transceiver 602 is in communications with CPE 604 and transceiver 606 is in communications with CPE 608. For the sake of example, the crosstalk from CO 606 and CPE 608 to either CO 602 or CPE 604 is described. However, it should be understood that interference may also be between the transmitter and receiver on the same subscriber line in both the upstream and downstream paths, which is the near-end echo of the transmit signal. The term “far-end” refers to when the source of interference is away from the receiving side and the term “near-end” refers to when the source of interference is close to the receiving side. For example, interference shown by arrow 612 illustrates noise generated by transceiver 606 coupled into the downstream communications and received by CPE 604. The term “victim” is applied to the line or the circuit being examined for crosstalk, and the term “disturber” is applied to the source of the crosstalk. Since the noise is generated away from the receiving side, this is referred to as downstream far-end crosstalk (FEXT). Likewise, interference shown by arrow 614 illustrates upstream near-end crosstalk (NEXT). Interference shown by arrow 616 illustrates upstream FEXT, and interference shown by arrow 618 illustrates downstream NEXT. In particular, downstream FEXT is a ubiquitous source of noise in VDSL. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies, such as mitigating downstream FEXT.